This invention relates to lubricant additives that provide anti-wear and friction-reducing properties when incorporated into lubricant compositions or other compositions where such properties are desired, such as motor oils.
A significant source of deterioration in machinery such as engines and motors that contain moving parts in mechanical motion at high temperatures is friction and wear between the contact surfaces of the moving parts. Such deterioration is particularly evident at startup and shutdown of the machinery. To combat these problems, lubricating agents such as lubricating oils, waxes and greases have traditionally been applied to the moving contact surfaces to prevent wear and to reduce friction.
Reducing or controlling friction is particularly important in motor oils, including automobile motor oils, because of the need to reduce wear, and also because this wear reduction must be accomplished while at the same time meeting standards for fuel economy as well as environmental vehicle fuel emissions control. Because of increased government regulation of vehicle fuel emissions, efforts have been made to improve engine performance, including improving engine design and emissions catalyst performance, as well as developing better additives such as lubricants and engine oil additives.
Ideally, a lubricant should provide lubrication of the entire contact surface. Such full-film contact is preferably achieved by completely coating the surfaces of the moving parts such that the parts never make contact. However, developing a full-film lubricant that is effective under the severe operating conditions of most engines and motors containing moving parts, has posed several difficulties. Design constraints, together with high load, slow speed, lubricant starvation, or low viscosity of the lubricant, may preclude full-film lubrication and increase the severity of contact. These conditions are often unavoidable during normal operation of machinery, and are particularly severe during startup and shutdown.
In cases where lubricants such as oils and greases cannot provide full-film lubrication at all times, anti-wear additives or friction modifiers are usually added. These anti-wear additives modify the surfaces to be lubricated through adsorption or chemical reaction to form coated surfaces that are characterized by reduced friction and increased wear resistance. It is generally recognized that different types of additives may interact in positive or negative ways and thereby enhance or interfere with each other""s performance. Antiwear agents and friction modifiers in particular, because they are believed to function by modifying the rubbing surfaces through adsorption or chemical reaction, have a high probability of affecting each other""s performance. This is because such materials adsorb on surfaces more or less strongly and compete with one another for surface adsorption sites. A strongly adsorbing material may exclude a more weakly adsorbing material from contact with the surface, thereby preventing it from exerting its effect on the surface. Such surface competition phenomena can pose significant challenges in developing additives and creating formulations where each additive can achieve its desired purpose.
Many kinds of anti-wear additives are known. In particular, organic phosphorus compounds such as dialkyl dithiophosphoric acids and dialkyl dithiophosphates have been used. Some of the most widely used and relied upon dialkyl dithiophosphates are metallic salts of dialkyl dithiophosphates, such as zinc dialkyldithiophosphates (ZDDPs), which find application in many different types of lubricants. The alkyl groups in zinc dialkyl dithiophosphates are typically derived from non-fluorinated alcohols that have been selected, based on chain length and degree of functional substitution, to impart desirable performance characteristics, such as solubility in the lubricant base fluid and thermal stability to the ZDDP. It is recognized that these characteristics can be changed by careful selection of the alkyl groups to optimize performance in particular applications.
ZDDP compositions are known to be effective in many formulations. This is evidence that they can compete very effectively for surface adsorption sites and thereby exert their effect on the rubbing surfaces. It might be predicted, therefore, that because ZDDPs adsorb strongly at surfaces and form very effective antiwear films by their chemical action at surfaces, such compounds would exclude other antiwear additives from adsorbing and exerting their effects at the surface.
Although ZDDPs have been used for many years in passenger car motor oil, their use is currently restricted because they contain phosphorus, and the amount of this element in motor oils is limited to less than 0.1%, since the phosphorus from ZDDP poisons catalytic converters, leading to increased vehicle emissions. It is anticipated that the future use of ZDDPs may be reduced even more than the current level. Anti-wear additives that can be used in place of ZDDPs, or in addition to them, are therefore of great interest.
ZDDPs have also been used in combination with certain molybdenum (Mo) additives, including soluble molybdenum additives such as molybdenum dialkyl dithiophosphates, molybdenum dialkyl dithiocarbamates and molybdenum amide complexes. One limitation of such ZDDP-Mo additive combinations, however, is that the molybdenum additives frequently reduce the anti-wear effectiveness of the ZDDPs, which is highly undesirable.
Other additives that may be included in lubricants as anti-wear additives include fluorinated organic compounds. Typical fluorinated compounds that may be used as lubricant additives include polytetrafluoroethylene (PTFE) and perfluoropolyether (PFPE). Fluorinated organic compounds, particularly esters and ethers, have been disclosed as lubricants for magnetic media, for example, in Japanese Patent 259482, Japanese Patent 08259501, and U.S. Pat. Nos. 5,578,387; 5,391,814 and 5,510,513.
Japanese Patent 01122026 teaches use of fluorine containing dibasic acid esters derived from diacids up to C8 as lubricants for magnetic media. This publication, as does PCT publication, US/92/08331, teaches that the acid structure from which the diester is formed may have double bonds present. The molecular structures taught by each of these publications may also have fluorine atoms present in each of the end group.
Partly-fluorinated adipic acid diesters, Rf(CH2)xO2C(CH2)4CO2(CH2)xRf, have been disclosed as lubricating coatings by Russian patent SU 449925. Bowers et al. (Lubr. Eng., July-August, 1956, pages 245-253) studied the boundary lubricating properties of several similar esters. The compounds disclosed in this publication have fluorine present in each of the diester groups, however the fluorination is symmetric. These symmetric, partially fluorinated esters have very low solubility in conventional lubricant base fluids and are therefore, of limited utility as additives in such base fluids.
Japanese Patent 2604186 discloses 1,2,3,4-butane-tetracarboxylic acid tetraesters with partly-fluorinated alcohols, but since all four ester groups are derived from fluorinated alcohols, these esters, too, are symmetric. Other examples of the teaching of symmetrically fluorinated molecular structures include U.S. Pat. Nos. 4,203,856; 5,066,412 and 4,039,301 and in JP08259482 and JP08259501.
Fluorine-containing tri-carbonyl compounds, including some esters, are disclosed as lubricant additives in Japanese patent JP 07242584, and partial fluoroesters of polycarboxylic acids, in which the acid functional groups are not completely esterified was taught in U.S. Pat. No. 3,124,533.
Fluorinated organic compounds are thought to protect metal surfaces from wear by forming metal fluorides on the coated surfaces. Surface studies of coated metal surfaces suggest that the fluorinated organic compounds undergo tribochemical reactions, which are friction-stimulated chemical reactions, with the metal surfaces to form the metal fluoride. For example, in the case of a steel mechanism, the surfaces of which have been lubricated with PTFE, deposits of iron fluoride have been observed in the near-surface region of the wear region. Metal fluorides such as iron fluoride are known to have good properties as solid lubricants, and, accordingly, it is hypothesized that the metal fluoride formed by the interaction of the PTFE and the metal shears more readily than the metal itself, and is less prone to weld-fracture type of wear. As a result, use of the PTFE reduces friction and wear in the mixed and boundary lubrication regimes, where actual contact between the moving surfaces may occur.
Although fluorinated materials such as those described above have been used as lubricant additives, there are certain limitations to their usefulness in these applications. One limitation of these fluorinated materials is their very low solubility in conventional lubricant base fluids such as natural and synthetic hydrocarbons and esters, which has effectively limited their application to use as solid additives. Although solid additives may be used in lubricants, they pose several problems.
For example, highly fluorinated organic compounds used as lubricants are generally insoluble in most conventional lubricant base fluids. For example, the high degree of insolubility of perfluoropolyethers (PFPEs) makes it extremely difficult to use them as additives in lubricant formulations. While PFPEs themselves can be used as the lubricant base fluid, their high cost makes such a modification prohibitively expensive. Similar insolubility problems are characteristic of polytetrafluoroethylene (PTFE). PTFE, which is a mostly insoluble solid, can be finely dispersed as particles in lubricant base fluids to reduce friction and wear. However, effectiveness of such a dispersed solid lubricant depends on maintaining the PTFE particles in stable dispersion. Achieving an indefinitely stable dispersion is a challenge, particularly in a formulated lubricant, which may contain detergents, dispersants, or surfactants that may destabilize the PTFE dispersion. Moreover, solid particles in suspension are not very effective at forming films on the contact surfaces of mechanical parts, and this reduces the effectiveness of the tribochemical reactions that must occur at the metal surface to provide the desired lubricity. This is in direct contrast to liquid or soluble materials that may adsorb onto the metal surfaces for which they have affinity, thereby modifying those surfaces directly by participating in the surface chemical reactions that provide the lubricating effect. Particles of a dispersed solid may also flocculate in the lubricant over time. Such flocculated particles may then plug or restrict flow of the lubricant in the equipment and result in lubricant starvation in critical locations.
In view of the deficiencies in the art, it is an object of the present invention to provide a fluorinated lubricant additive which can serve as an anti-wear agent and friction reducer, and which, moreover, is compatible with conventional lubricant base fluids typically used in lubricant compositions. Desirably, such a lubricant additive should also overcome the cost and solubility limitations of previously known fluorinated organic compounds. This object has been achieved by the fluorinated compounds and compositions of the present invention.
The present invention provides fluorinated organic compounds according to formula (I), or metallic salts thereof: 
wherein R1 and R2 are each independently selected from the group consisting of C1 to C40 organic residues; and
wherein R1 and R2 are different, or R1 and R2 form a ring, and at least one of R1 and R2 is a fluorinated C1 to C40 organic residue.
Another embodiment of the invention comprises a compound of formula (I), or metallic salts thereof: 
wherein R1 and R2 are each independently selected from the group consisting of C1 to C40 organic residues; and
wherein R1 and R2 are the same or different, or R1 and R2 form a ring, and at least one of R1 and R2 is a fluorinated C1 to C40 organic residue, provided that when R1 and R2 are the same, neither R1 nor R2 can be xe2x80x94(CH2(CF2)xCF2H), where x is 1, 3 or 5.
The invention further comprises an anti-wear additive comprising a compound of formula (I), or metallic salts thereof: 
wherein R1 and R2 are each independently selected from the group consisting of C1 to C40 organic residues, or R1 and R2 form a ring; and
wherein at least one of R1 and R2 is a fluorinated C1 to C40 organic residue.
The present invention also provides a process of making an anti-wear additive comprising:
a) preparing a mixture of two or more compounds, wherein said mixture includes at least one fluorinated compound and at least one non-fluorinated compound;
b) reacting the mixture with a thiophosphorus compound to form one or more oxygen linkages between the phosphorus atom of the thiophosphorus compound and each of the fluorinated and non-fluorinated compounds; and
c) recovering a fluorinated dithiophosphoric acid compound having the molecular structure according to formula (I): 
xe2x80x83wherein R1 and R2 are each independently selected from the group consisting of fluorinated C1 to C40 organic residues; and
xe2x80x83wherein R1 and R2 are different, or R1 and R2 form a ring.
When prepared in this way, the compounds of the present invention are generally produced in admixture with compounds where both R1 and R2 are fluorinated and with other compounds where both R1 and R2 are non-fluorinated. It is generally not necessary to separate or purify the compounds of the present invention when they are produced in such mixtures, and they may be used in that form in various applications.
The process of making anti-wear additives according to the invention may also include reacting the product of formula (I) with a source of metal atoms to form a metallic salt.
In yet another embodiment, the present invention includes a composition comprising a lubricant base fluid and one or more fluorinated anti-wear additives according to formula (I), and/or a metallic salt thereof, wherein R1 and R2 are each independently selected from the group consisting of C1 to C40 organic residues; and further wherein at least one of R1 and R2 is a fluorinated C1 to C40 organic residue.